Dabi

SPOILER WARNING | END OF THE MANGA

Dabi happens to be the oldest son of Enji Todoroki, also known as Endeavor, the ex number one hero. His real name is Toya Todoroki, when he was 13 years old, he asked his father to meet him on the Sekoto hill to show him his new abilities. Enji won't come and Toya will loose the control of his powers, leading to a 3 years coma. His entire family, composed of his two younger brothers Natsuo and Shoto Todoroki, his mother Rei Todoroki and his younger sister, Fuyumi Todoroki, will consider him dead, having the fact that they found his jaw on the Sekoto hill. 

When he wakes up he firstly want's to go back to his home but when he arrives Enji is training Shoto, his youngest brother, a way that made him realize, Enji is not a good person. He then will learn to control his powers and become Dabi, the villain we know. 

Years later, when Shoto enters to UA, he joins the League of villains, and having in mind the objective of killing Endeavor, his father, he will get to the top of the villains and will show his true strength. Later on, when Endeavor has become the number 1 hero of Japan, he will reveal his true identity and he attempted to kill his father as well a his little brother Shoto. Stopped by Best Jeanist, he will try again, leading to a deadly heart injury. 

After the 8 years ellipse, we won't have any news of him, but just before, Horikoshi let us understand that he would die after a short amount of time due to his injuries. 

If he survived, he would be 32 years old by the time of the end of the manga.